1. Field of the Invention
The present invention relates to the radiation of objects by ionizing particles and/or ionizing radiation generated from an electron linear accelerator more commonly referred to as an electron gun, and more particularly aims at the real time control, i.e., during the ionization, of the dose rate of the radiations applied to objects.
Such radiations and particle beams are commonly used for the polymerization or cross-linking of resins, directly by the action of the electron beam generated by the gun, or indirectly by an X-ray radiation generated by a target placed in the electron beam, of photocrosslinkable resins involving a composite material, or to ensure the sterilization, by means of X-rays or electrons, for the purpose of preserving foodstuffs or the sterilization of medical instruments.
2. Background and Material Information
Although the present invention is subsequently described in its application to the polymerization of resins involving composite materials constituting all or part of the treated element or structure used especially in the fields of automobile, aeronautics or space. Other fields of application are involved or envisioned, such as the sterilization and/or the preservation of foodstuffs or other products or objects, in particular.
In the field of polymerization by ionization of composite materials, one uses an electron gun associated with a retractable tungsten target which, when interposed in the electron beam of the gun, makes it possible to generate a secondary X-ray radiation.
The electron irradiation of the element constituted, totally or partially, of a composite material, namely, resin-impregnated mineral or synthetic fibers, is carried out for composite thicknesses less than about 25 mm, whereas the X-rays are used for greater thicknesses up to about 300 mm.
Although control of the power of the electron accelerator is effected prior to any polymerization operation, in a very accurate manner, by means of a retractable calorimeter, this control is not feasible during the ionization, and there is currently no means for controlling the dose rate sent on the composite material.
It is fundamental to subject the material to the action of a sufficient dose of radiation, in order to suitably cross-link the resins or the radiation-hardenable products falling within the composition of the element to be treated. This dose is the quantity of energy that is necessary to completely cross-link or polymerize the treated product. It defines a general effect, independently of the time required to produce it. It is therefore necessary to properly determine and control the dose rate that characterizes the effect produced in the time unit, as well as the time for applying this dose rate.
This problem related to the dose rate control is particularly crucial when using the X-ray radiation. x-ray radiation allows for penetrations that are much more substantial than those of the electron radiation, but overdoses the surface of the element to be treated, if one desires to obtain the sufficient dose of irradiation at a given depth.
Furthermore, the necessary doses for the polymerization or cross-linking of resins of the element to be treated are resin-dependent and vary to a large extent, for example between 2 and 10 Mrad, the desired dose being obtained by playing with the two factors: time and dose rate.
The conventional industrial process for irradiating an element made of a composite material consists of determining, from charts taking into account the geometry and the nature of the element, a polymerization cycle during which the profile of the irradiation curve, i.e., the variations of the dose rate, will develop.
However, the above-mentioned prior control of the power of the electron accelerator does not guarantee, especially in the case of an X-ray irradiation, that the necessary doses will be actually sent to the various portions of the element along the profile of the pre-established irradiation curve, because variations or "misfires", due to missing pulses, can occur in the power of the electron gun.
The quality control of the conventional process consists of verifying, after the irradiation of the element, that any point located in the thickness of the element has received at least a certain dose of radiation, this verification being carried out, in the known manner, by various systems of dosimetric films.
This is a method of control a posteriori i.e., from the cause to the effect that is reasonably accurate and reliable, but it would be more advisable to be able to control the dose rate values in real time, i.e., during the ionization of the element, in order to verify that the irradiation profile of the polymerization cycle is followed, so as to carry out the quality control of the element while irradiating it, thus avoiding the traditional control a posteriori and, in particular, to possibly rectify the local irradiation acting on the electron gun to prevent a temporary drop in the operation mode, for example.